cfg80211: fix NETDEV_UNREGISTER notifier

It's possible to get the NETDEV_UNREGISTER callback multiple
times (see net/core/dev.c:netdev_wait_allrefs) and this will
completely mess up our cleanup code. To avoid that, clean up
only when the interface is still on the wiphy interface list
from which it's removed on the first NETDEV_UNREGISTER call.
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/wireless/core.c

@@ -725,15 +725,22 @@ static int cfg80211_netdev_notifier_call(struct notifier_block * nb,
 		break;
 	case NETDEV_UNREGISTER:
 		mutex_lock(&rdev->devlist_mtx);
+		/*
+		 * It is possible to get NETDEV_UNREGISTER
+		 * multiple times. To detect that, check
+		 * that the interface is still on the list
+		 * of registered interfaces, and only then
+		 * remove and clean it up.
+		 */
 		if (!list_empty(&wdev->list)) {
 			sysfs_remove_link(&dev->dev.kobj, "phy80211");
 			list_del_init(&wdev->list);
-		}
-		mutex_unlock(&rdev->devlist_mtx);
 			mutex_destroy(&wdev->mtx);
 #ifdef CONFIG_WIRELESS_EXT
 			kfree(wdev->wext.keys);
 #endif
+		}
+		mutex_unlock(&rdev->devlist_mtx);
 		break;
 	case NETDEV_PRE_UP:
 		if (!(wdev->wiphy->interface_modes & BIT(wdev->iftype)))